Silicon-Germanium (SiGe) transistors are high-speed, low cost transistors. They are produced by growing a thin, epitaxial layer of Ge on a Si substrate. Because of the 4% difference in size between the Si and Ge lattice structures, the Si lattice near the surface of the Si substrate is strained, allowing material parameters such as the band gap to be varied. In particular the strained Si has increased carrier mobility (2–3 time greater than Si), allowing faster switching-speed transistors to be fabricated. Moreover, because the Ge is a thin, epitaxial layer, standard Si fabrication technology can be used, resulting in low costs and providing compatibility with conventional CMOS technology.
The high speed switching of SiGe bipolar transistors, however, comes at the cost of reduced transistor breakdown voltage and increased intrinsic capacitance of the transistors. These drawbacks become particularly troublesome when attempting to implement SiGe class A high-power amplifiers capable of operating at high frequencies (greater than 1 GHz). In such amplifiers, the power output is proportional to the product of the current and the voltage, i.e., power=(Vrms·Irms)/2. The reduced break down voltage, therefore, significantly reduces the maximum power that can be output from an amplifier using SiGe transistors. In addition, the high intrinsic capacitance of the transistors results in significant negative feedback of current at high frequencies (greater than 1 GHz), further reducing the power output of such amplifiers. And finally, the lack of via holes causes high inductive feedback which greatly decreases gain.
What is needed is a method and apparatus that enables high-speed, low-cost SiGe bipolar transistors to be effectively used in high-speed power amplifiers.